The disclosure relates to:
(a) a foundry mixture that consists of a dry, granular refractory material (typically sand), a binder, optional additives, and a cleaning agent, and
(b) a method for molding and/or electrolytic cleaning of a metal part molded in a mold formed from the foundry mixture.
A common manufacturing method for the production of metal parts is foundry casting. Metal castings are cast in molds or receptacles formed from a conventional foundry mixture consisting of granulated foundry sand and a cured binder. The granulated sand takes the desired shape of the mold and the cured binder enables the granulated sand to retain the shape of the mold. The mold includes a shell defining a mold cavity. The mold may optionally include one or more cores placed in the mold cavity to define hollow elements or passages in the cast metal part, with the shell and cores defining the shape of the casting. Liquid metal is poured into the mold cavity and solidifies upon cooling to form the casting. The solid casting is then removed from the mold.
Some binders may include a binder material that is treated to hold or bond the refractory material within a rigid binder matrix. Other binders may include a compatible suspension agent along with the binder material that reacts with or otherwise cooperates with the binder material to hold or bond the foundry sand within a rigid binder matrix.
One type of binder includes a resin as the binder material and may utilize a suitable catalyst as the suspension agent. The resin cures to form a cured resin matrix. Resins commonly used as binder materials include (but are not limited to) urea formaldehyde (UF), phenol formaldehyde (PF) resins, and natural or synthetic gums.
Binders that form a cured resin matrix are referred to as “resin binders” herein. Resin binders may use a resin alone (that is, the resin binder does not include a suspension agent) or may include a resin and a catalyst as suspension agent.
The resin may be a thermosetting resin or heat-cured resin that cures or cross-links when heated, or the resin may require the presence of a catalyst to induce curing or cross-linking of the resin. When the foundry mixture is formed, the resin is treated to cure the resin. Specific resins use different types of treatment to form the matrix. “Hot-box”, “cold-box”, and “no-bake” are examples of different treatment types.
Hot-box treatment utilizes pre-heating the foundry mixture with a thermosetting resin binder. The foundry mixture is typically heated to temperatures between about 35 degrees Centigrade and about 300 degrees Centigrade to cure the resin. Resins used in hot-box treatment may include furan resins and furfuryl alcohols. Typically the resins are cured in the presence of a latent acid curing catalyst.
Cold-box treatment utilizes passing a vapor or gas catalyst through the foundry mixture to induce curing of the resin. The resin used is typically a phenolic urethane. A gaseous tertiary amine curing catalyst is passed through the shaped sand and resin mixture to cure the mixture. The catalyst may be TEA (tetraethylamine) and DMEA (dimethylethylamine). The sand and resin mixture may be shaped in a pattern and allowed to cure and become self-supporting to form the mold.
No-bake treatment utilizes a catalyst added directly to the resin that cures the resin at ambient temperatures without the need for baking. The resin used is typically a phenolic urethane. The suspension agent includes a solvent that reacts with a liquid curing catalyst mixed with the sand and resin before shaping. The foundry mixture typically cures 30 minutes to a few hours after mixing in the solvent.
Binders that do not utilize a resin are referred to as “non-resin binders” herein.
Some types of non-resin binders utilize water or some other liquid (such as vegetable oil, marine oil, or other liquids known in the art) as a suspension agent that binds the binder material together. Non-resin binders that utilize water or other liquid as a suspension agent are referred to as “liquid cured binders” herein. Liquid cured binders that utilize water as a suspension agent are referred to as “aqueous binders” herein, while binders that do not utilize water as a suspension agent are referred to as “non-aqueous binders” herein. Resin binders that are heat cured or catalyst cured, for example, are non-aqueous binders.
Some types of aqueous binders include clays (such as bentonite or kaolinite) or other solid mineral agent as the binder material. The sand, mineral agent, and water are mixed together. There is sufficient water and time after mixing to hydrate the binder material and form a mortar. The mortar dries and becomes rigid, thereby holding the sand within a mortar matrix.
Some aqueous binders utilize calcium oxide, CaO, as a precursor binder material. The calcium oxide reacts with the water suspension agent to form a calcium hydroxide mortar. There is effectively no calcium oxide in the foundry mixture after the calcium oxide has hydrated and the binder has cured.
Some types of binders include a non-resin binder material that is cured by heating. Such binders are referred to as heat-cured non-resin binders herein.
One type of heat-cured non-resin binder includes inorganic clay components such as aluminum silicate, bentonite, or montmorillonite as a binder material. In embodiments the clay is heated to form a clay binder matrix that holds the sand within the clay matrix.
Yet other types of non-resin binders include sodium silicate as a binder material.
Binders in a foundry mix in which the binder material has been treated to form the binder matrix are referred to as “cured binders” herein. Cured binders include cured resin binders in which the resin has been cured by heating or by catalyst reaction to form a resin binder matrix, cured liquid cured binders in which the binder material has been mixed with a liquid and reacts to form a cured binder matrix, and heat-cured binders in which the binder material has been heated to form a cured binder matrix.
The foundry mixture may also optionally include additional material or materials to improve the finish of casting surfaces, the dry strength of the mold, refractoriness, and “cushioning” (the creation of voids in the mold that enable the mold to expand when metal is poured into the mold), or to provide other desirable characteristics in the finished mold.
Typically, up to 5% of reducing agents, such as coal powder, pitch, creosote, and fuel oil, may be added to the foundry mixture to prevent wetting (liquid metal sticking to sand particles, thereby leaving sand particles on the casting surface), improve surface finish, decrease metal penetration, and burn-on defects. These additives achieve this by creating gases at the surface of the mold cavity, which prevent the liquid metal from adhering to the sand.
Typically, up to 3% of “cushioning material”, such as wood flour, saw dust, powdered husks, peat, and straw, can be added to the foundry mixture to reduce scabbing, hot tear, and hot crack casting defects when casting high temperature metals. These materials burn-off when the metal is poured, thereby creating voids in the mold that allow the mold to expand.
Typically, up to 2% of cereal binders, such as dextrin, starch, sulphite lye, and molasses, can be used in the foundry mixture to increase dry strength (the strength of the mold after curing) and improve surface finish. Cereal binders also improve collapsibility and reduce shakeout time because they burn-off when the metal is poured.
Typically, up to 2% of iron oxide powder can be used in the foundry mixture to prevent mold cracking and metal penetration, essentially improving refractoriness. Silica flour (fine silica) and zircon flour may also improve refractoriness.
Material or materials added to the foundry mixture to improve the finish of casting surfaces, the dry strength of the mold, refractoriness, and/or cushioning are referred to as “additives” herein.
After casting, sand and binder still adhering to casting surfaces are typically removed by mechanical agitation of the casting, shot blasting, or other mechanical cleaning methods. Alternatively, the casting may be dipped into a molten bath.
Used sand cleaned from the casting has economic value. Used foundry sand is, for example, used as a fine aggregate in making concrete.
Often after mechanical cleaning of the casting or removal of the casting from the molten bath, some sand and binder remains adhering to casting surfaces. Removal of this remaining foundry mixture is often difficult and time consuming.
Hathaway US Patent Application Publications 20050087323 and 20050087321 each disclose a foundry mixture that includes sand, a resin binder, and a disintegration additive that reportedly assists in removing the foundry mixture from casting surfaces. The casting is electrolytically cleaned after being removed from the mold. The disintegration additive assists during the electrolytic cleaning in removing the remaining foundry mixture adhering to casting surfaces.
The disintegration additive is a salt that is preferably inorganic and soluble in water. Preferred embodiments of the mixture include disintegration additives having relatively high melting points (above 300 degrees C., which is much lower than the melting points of common cast metals such as iron, steel, titanium, or aluminum).
Specific examples of disintegration additives are given in paragraph 22 of the '323 publication. Preferred anions for the salt of the disintegration additives include carbonates, nitrates, sulfates, phosphates, hydroxides, and halogens. Certain preferred salts include cations of sodium, potassium, calcium, ammonium, or magnesium, and include salts, such as for example: sodium carbonate, sodium bicarbonate, sodium chloride, sodium hydroxide, sodium iodide, sodium nitrate, sodium phosphate, disodium phosphate, sodium sulfate, potassium carbonate, potassium chloride, potassium hydroxide, potassium iodide, potassium nitrate, potassium phosphate, potassium sulfate, calcium carbonate, calcium chloride, calcium hydroxide, calcium iodide, calcium nitrate, calcium sulfate, ammonium sulfate, ammonium carbonate, magnesium carbonate, magnesium chloride, magnesium hydroxide, magnesium iodide, magnesium nitrate, magnesium phosphate, magnesium sulfate, and equivalents and mixtures thereof. The disintegration additive may be selected from the group consisting of sodium chloride, potassium chloride, sodium carbonate, sodium bicarbonate, sodium phosphate, and mixtures thereof. The disintegration additive may comprise sodium chloride. The disintegration additive may comprise sodium bicarbonate, disodium phosphate, and mixtures thereof. The disintegration additive may comprises sodium carbonate, disodium phosphate, and mixtures thereof.
Hathaway discloses in embodiments that the disintegration additive reportedly enhances the electron ion conduction of the casting when contacted with a polar electrolyte such as water. Water soluble salts would be suitable for such disintegration agents.
Hathaway discloses in other embodiments that the disintegration additive volatilizes during casting of the metal part, leaving behind a porous and slightly unstable mold structure. Hence, the melting point of such disintegration agents must be below the melting point of the metal being cast.
It has been found, however, that volatizing the disintegration additive during casting may adversely impact mold strength, and may adversely impact the finish of the casting surfaces. Furthermore, some disintegration additives include sodium that impairs the economic value of used, recovered sand. The sodium contaminates the used sand, making the sand unsuitable as a fine aggregate in concrete.